Thanks for the advice..

If it's off the power circuit, you probably need an FCU for the
unmentionable item you're fitting anyway.

Why is a mirror with lights unmentionable?

There are two fluorescent tubes behind the glass of the mirror, and they
shine through a unmirrored area. They are low power, and give off enough
light so that I dont need to use the main bathroom lights on a cold dark
winter morning, so it is not like they are a vulgar waste of electricity.

However...

The mirror also has one of those 25w heating pad on the back that stop it
from misting up. I first saw these in Japan about 5 years ago and was really
impressed, and they do work exceptionally well. When i went to the shop to
buy the pad, the yound Saturday boy serving me said "I dont see the point of
these, all you have to do iw wipe the mirror with a towel. Clearly he didnt
shave at the time, nor did he have to clean his own bathroom mirror ;-)

If the objection is to do with power consumption, maybe i'll not start a
thread about under-tile heating (150w/m^-2) :-)

Cheers,
Chris